Many different devices and methods are presently being used to measure the concentration of various sugars and amino acids in a wide variety of solutions. Many industrial manufacturing and food processing systems require that the level of one or more sugars and/or amino acids be carefully monitored at various stages to help insure the desired quality of final products. For example, the varying glucose concentrations during fermentation processes are important process control parameters, and their continuous monitoring can improve the yield and quality of the fermentation product. In addition, there are a large number of situations where the amount of sugar in finished food stuffs and other sugar containing products must be determined. On line continuous measurement is important in reducing the risk of contamination, labor costs and delays associated with off-line measurements. To be able to perform on-line measurements, there is a need for sterilizable sensors with rapid response times and high sensitivity, yet which also require minimum maintenance and calibration.
One important use for sugar analysis techniques is in the medical field where monitoring of sugar levels in biological fluids is desirable for the proper diagnosis and treatment of diabetes and other diseases. With respect to medical applications, glucose is by far the most important sugar, and diabetes is the most common disease for which glucose determinations are routinely conducted. Diabetes is a disease of the metabolic system that affects more than 14 million people in the United States and over 100 million people worldwide. It is characterized by an elevated blood-glucose concentration which is caused by a lack of the hormone insulin. Sugars are the primary source of metabolic energy, and the inability to self-regulate the levels of sugar metabolized by the body leads to many other medical problems, including but not limited to blindness, heart disease and kidney failure.
The one-time or ‘spot’ measurement of blood-glucose concentration ex vivo is also not optimal, as it requires collection of a blood sample, usually obtained by pricking the finger, which must be done at least several times each day. To avoid the need to subject the patient to this painful process for glucose monitoring, much effort has gone into identifying alternative sources for samples, such as subcutaneous tissue fluid, urine or saliva instead of blood, and for identifying less-painful and more efficient ways to obtain these samples, such as by transdermal extraction or using very thin needles. The development of new, highly sensitive and miniaturizable glucose monitoring technology will make some of these alternative methods more feasible for use at home by patients.
Treatment of diabetes involves monitoring of the patient's blood-glucose levels, with insulin injections being given when the glucose concentration rises above normal levels. A simple and accurate method for measuring blood-glucose concentrations is an essential cornerstone of any diabetes treatment protocol, since excessively low blood-glucose levels in diabetes patients can result in coma and even death. Frequent testing and insulin administration can significantly reduce long-term complications of diabetes. The vast majority of sensors which are used currently for glucose monitoring are based on enzymes such as glucose oxidase or glucose dehydrogenase. These enzyme-based sensors are simple to use and have relatively high sensing selectivity. They are widely used for one-time measurement of blood-glucose concentrations ex vivo. However, among the many drawbacks of enzyme-based sensors are that they are costly and have a short life time. This inherently unstable enzyme must be protected from extreme conditions during manufacturing and storage in order to preserve its catalytic activity. In addition, there have been a number of problems associated with the use of enzymes in implantable sensors used in systems for continuously monitoring blood-glucose levels in vivo, among them the fact that enzymes can elicit an immune response and are not stable to most sterilization methods. The development of non-enzymatic approaches to glucose sensing is desirable in order to provide more effective management of diabetes, both for spot monitoring of glucose concentrations as well as for in vivo continuous monitoring.
A few nonenzymatic methods for measuring glucose have been proposed as alternatives to the above-described enzyme-based devices. U.S. Pat. No. 5,217,691 describes the use of boronic acids for the semiquantitative colorimetric determination of glucose. U.S. Pat. No. 4,371,374 discloses separating and quantitating glycosylated amino acids, peptides or mixtures thereof by treating a urine sample with a suitable boronic acid to complex the glycosylated compounds, separating them and analyzing the separated complexed material. U.S. Pat. No. 5,503,770 discloses a fluorescent boronic acid conjugate which emits fluorescence upon binding to saccharides. U.S. Pat. No. 5,244,562 discloses switching devices coated with a polymer imprinted with glucose and boronic acid.
Although some of the above-described sensors have shown promise, none have been found to be entirely satisfactory. There is a continuing need to develop robust sensor systems that can be used to simply, accurately, and rapidly measure the amount of a particular sugar, amino acid or related compound which is present in solutions and other environments. The need is particularly apparent with respect to ex vivo and in vivo glucose determinations which can be an important component in medical diagnosis and treatment of various metabolic disorders, including diabetes. There is also a strong need to develop robust sensor technology for real-time monitoring of sugars, amino acids and other metabolites in manufacturing process environments.